# Liquid level measurement (Wet leg and Dry leg)

By | January 12, 2016

Differential pressure transmitters used for liquid level applications measure hydrostatic pressure head. Liquid level and specific gravity of a liquid are factors in determining pressure head. This pressure is equal to the liquid height above the tap multiplied by the specific gravity of the liquid. Pressure head is independent of volume or vessel shape.

Open Vessels :- A pressure transmitter mounted near a tank bottom measures the pressure of the liquid above.Make a connection to the high pressure side of the transmitter, and vent the low pressure side to the atmosphere. Pressure head equals the liquid’sspecific gravity multiplied by the liquid height above the tap. Zero range suppression is required if the transmitter lies below the zero point of the desired level range. Figure  shows a liquid level measurement example.

Closed Vessels :-  Pressure above a liquid affects the pressure measured at the bottom of a closed vessel. The liquid specific gravity multiplied by the liquid height plus the vessel pressure equals the pressure at the bottom of the vessel. To measure true level, the vessel pressure must be subtracted from the vessel bottom pressure. To do this, make a pressure tap at the top of the vessel and connect this to the low side of the transmitter. Vessel pressure is then equally applied to both the high and low sides of the transmitter. The resulting differential pressure is proportional to liquid height multiplied by the liquid specific gravity.

Dry Leg Condition
Low-side transmitter piping will remain empty if gas above the liquid does not condense. This is a dry leg condition. Range determination calculations are the same as those described for bottom-mounted transmitters in open vessels, as shown in Figure

Liquid LevelMeasurement Example:-

Let X equal the vertical distance between the minimum and maximum measurable levels (500 in.).
Let Y equal the vertical distance between the transmitter datum line and theminimum measurable level (100 in.).
Let SG equal the specific gravity of the fluid (0.9).
Let h equal the maximum head pressure to be measured in inches of water.
Let e equal head pressure produced by Y expressed in inches of water.
Let Range equal e to e + h.
Then h = (X)(SG)
= 500 x 0.9
= 450 inH2O
e = (Y)(SG)
= 100 x 0.9
= 90 inH2O
Range = 90 to 540 inH2O Wet Leg Condition :-
Condensation of the gas above the liquid slowly causes the low side of the transmitter piping to fill with liquid. The pipe is purposely filled with a convenient reference fluid to eliminate this potential error. This is a wet leg condition. The reference fluid will exert a head pressure on the low side of the transmitter. Zero elevation of the range must then be made. See Figure

Let X equal the vertical distance between the minimum and maximum measurable levels (500 in.).
Let Y equal the vertical distance between the transmitter datum line and the minimum measurable level (50 in.).
Let z equal the vertical distance between the top of the liquid in the wet leg and the transmitter datum line (600 in.).
Let SG1 equal the specific gravity of the fluid (1.0).
Let SG2 equal the specific gravity of the fluid in the wet leg (1.1).
Let h equal the maximum head pressure to be measured in inches of water.
Let e equal the head pressure produced by Y expressed in inches of water.
Let s equal head pressure produced by z expressed in inches of water.
Let Range equal e – s to h + e – s.
Then h = (X)(SG1)
= 500 x 1.0
= 500 in H2O
e = (Y)(SG1)
= 50 x 1.0
= 50 inH2O
s = (z)(SG2)
= 600 x 1.1
= 660 inH20
Range = e – s to h + e – s.
= 50 – 660 to 500 + 50 – 660
= –610 to –110 inH20  